Primitives and Commands
OpenGL draws primitivespoints, line segments, or polygonssubject to several selectable modes. You can control modes independently of one another. That is, setting one mode doesn't affect whether other modes are set (although many modes may interact to determine what eventually ends up in the framebuffer). To specify primitives, set modes, and perform other OpenGL operations, you issue commands in the form of function calls.
Primitives are defined by a group of one or more vertices. A vertex defines a point, an endpoint of a line, or a corner of a polygon where two edges meet. Data (consisting of vertex coordinates, colors, normals, texture coordinates, and edge flags) is associated with a vertex, and each vertex and its associated data are processed independently, in order, and in the same way. The only exceptions to this rule are cases in which the group of vertices must be clipped so that a particular primitive fits within a specified region. In this case, vertex data may be modified and new vertices created. The type of clipping depends on which primitive the group of vertices represents.
Commands are always processed in the order in which they are received, although there may be an indeterminate delay before a command takes effect. This means that each primitive is drawn completely before any subsequent command takes effect. It also means that state-querying commands return data that is consistent with complete execution of all previously issued OpenGL commands.